Correctly support form POST with text/plain content type

Fixes #279

Author: mathieucarbou

src/WebRequest.cpp

@@ -185,7 +185,10 @@ void AsyncWebServerRequest::_onData(void *buf, size_t len) {
             _isPlainPost = true;
           } else if (_contentType == T_text_plain && __is_param_char(((char *)buf)[0])) {
             size_t i = 0;
-            while (i < len && __is_param_char(((char *)buf)[i++]));
+            char ch;
+            do {
+              ch = ((char *)buf)[i];
+            } while (i++ < len && __is_param_char(ch));
             if (i < len && ((char *)buf)[i - 1] == '=') {
               _isPlainPost = true;
             }